There are known pulse modulator systems which use energy storage devices as part of a pulse forming network (PFN) to store energy during a charging cycle, with the stored energy being released during a discharge cycle and dissipated in a load through use of a switching arrangement.
A known pulse modulator used for obtaining high voltage square pulses such as are used in radar transmitters is a line type pulser, or "soft tube modulator". In these systems, the PFN consists of a series of lumped capacitances and impedances, with the dominant effect being that of a capacitance during the charging cycle. All of the energy stored in the PFN is dissipated when the PFN is dumped into the load on closing of the pulser electronic switch, such as a thyratron or SCR. The problems with this type of pulse modulator are that it is impossible to change the pulse width from pulse to pulse, it is necessary to use a high voltage power supply, and impedance matching of the load is required in many cases.
In pulse modulator systems where an inductor is the energy storage device employed in the PFN, storage and dissipation of the energy in the inductor is determined by the corresponding exponential rise and decay of the current flowing therein. The time constant of the circuit determines the pulse width during which all of the stored energy is dissipated. Since the time constant does not change, if a shorter charging time is used, the necessary result is a limitation of the amplitude of the current available to the load. In some applications this may become a problem, as it is impossible to maintain a constant pulse amplitude.
For example, in the case of a car ignition system where an induction coil is used for storage of energy, increased PRF is the result of increased engine rpm. The undesired effect is that the amount of stored energy available for the spark is decreased, and this may adversely affect performance as the spark gap becomes fouled and greater pulse amplitudes are required to ensure spark generation.
Another type of pulse modulator used for obtaining high voltage square pulses is a "hard tube modulator". In this type of modulator, a large capacitor in which energy has been stored is partially discharged by a high voltage vacuum switch (pulse triode) into the load. In this modulator, it is possible to vary the pulse width, but because of the large capacitance required, the weight is considerably increased. In addition, due to the high impedance of the vacuum switch, a low efficiency is achieved.
It would therefore be desirable to provide a current pulse generator where the pulse width is variable from pulse to pulse and a high PRF can be achieved with high efficiency and little or no change in the output amplitude, without the requirement of a high voltage power supply or for impedance matching.